Na-K-ATPase has been found to be different in various segments of rabbit nephron from 0 to 200 pmole/mm tubule length/minute associated with the difference in the rates of Na ion reabsorption in various segments. Furthermore, Na-K-ATPase activity was found to be stimulated in cortical collecting ducts by pretreatment of rabbit with DOCA as in the case with Na reabsorption and K secretion.